Ordered Mesopore Confined Pt Nanoclusters Enable Unusual Self-Enhancing Catalysis

As an important kind of emerging heterogeneous catalyst for sustainable chemical processes, supported metal cluster (SMC) catalysts have received great attention for their outstanding activity; however, the easy aggregation of metal clusters due to their migration along the substrate’s surface usually deteriorates their activity and even causes catalyst failure during cycling. Herein, stable Pt nanoclusters (NCs, ∼1.06 nm) are homogeneously confined in the uniform spherical mesopores of mesoporous titania (mpTiO2) by the interaction between Pt NCs and metal oxide pore walls made of polycrystalline anatase TiO2. The obtained Pt-mpTiO2 exhibits excellent stability with well-retained CO conversion (∼95.0%) and Pt NCs (∼1.20 nm) in the long term water–gas shift (WGS) reaction. More importantly, the Pt-mpTiO2 displays an unusual increasing activity during the cyclic catalyzing WGS reaction, which was found to stem from the in situ generation of interfacial active sites (Ti3+-Ov-Ptδ+) by the reduction effect of spillover hydrogen generated at the stably supported Pt NCs. The Pt-mpTiO2 catalysts also show superior performance toward the selective hydrogenation of furfural to 2-methylfuran. This work discloses an efficient and robust Pt-mpTiO2 catalyst and systematically elucidates the mechanism underlying its unique catalytic activity, which helps to design stable SMC catalysts with self-enhancing interfacial activity in sustainable heterogeneous catalysis.


■ INTRODUCTION
The heterogeneous catalytic reaction is an important sustainable chemical process in industry production, and such a process is strongly susceptible to the surface and interface properties (e.g., acidity/basicity, active sites, coordination states) and micro/nano structure (e.g., size, facets, defects) of catalysts. 1−4 Tailoring the property of nanosized metals at the interfacial sites of heterogeneous catalysts is crucial to forming excellent active sites and improving the catalyst efficiency. 4−7 Thereby, the interfacial sites induced by metal−support interaction (MSI) play a decisive role in most heterogeneous catalytic processes, such as CO oxidation, 8,9 methanol oxidation, 10 CO 2 reduction, 11−13 and the water−gas shift (WGS) reaction. 14−16 In terms of the WGS reaction, which is vital for eliminating carbon monoxide and solving the energy crisis, the reaction between carbon monoxide and water to produce clean energy hydrogen is strongly dependent on the interfacial sites and active species.
Recently, considerable attention has been paid to building strong metal−support interaction (SMSI) systems using reducible metal oxide supports, aiming to develop stable heterogeneous catalysts. 17−23 However, SMSI can lead to overencapsulation of metal nanoparticles (NPs) by the reducible metal oxide supports, especially for large metal NPs of several nanometers, eventually impeding the exposure of interfacial active sites to achieve high catalytic efficiency. 24 Suppression of the size of metal nanoparticles at the support's surface was demonstrated to be a reliable approach to realizing an appropriate encapsulation of metal NPs and generate a favorable metal/support interface. 25−31 Such a metal/support interface was originally found in CeO 2 particle-supported Pt catalyst 27 and later defined as the electronic metal−support interaction (EMSI). 28 The EMSI can induce a strong electronic perturbation between ultrafine metal nanoclusters of less than 2 nm and reducible metal oxides, contributing to excellent catalytic activity. 29−31 Ma and co-workers have fabricated series of stable supported metal cluster (SMC) catalysts and clarified their advantages in various important catalytic fields. 32−35 Designed construction of an appropriate MSI is extremely important to creating SMC catalysts with high stability and activity. 36 The stable interfacial sites with an appropriate MSI allow for excellent performance toward the WGS reaction that is widely used in the chemical industry (e.g., production and purification of H 2 ) due to the accelerated H 2 O dissociation and intermediate transfer. 37−43 To date, various methods have been reported for the synthesis of SMC catalysts using nonporous or porous supports. 31,44,45 Nonporous supports cannot guarantee the long-term stability of SMC catalysts because of the inevitable aggregation of metal NCs especially at high working temperatures. 46 Microporous materials (pore size <2 nm) including zeolites can exert a nanoconfinement effect for NCs to ensure the stability by preventing aggregation; 47−50 however, the resultant SMC catalysts mainly provide dominant surface diffusion of reactant molecules during heterogeneous catalysis, which is unfavorable for catalysis due to the limited accessibility of active sites. By contrast, mesoporous materials have larger uniform nanopores of 2−50 nm, which are comparable to the free mean paths of gaseous molecules, and tunable pore wall microenvironments (e.g., active sites, surface acidity, surface reducibility). 51−53 The highly interconnected mesopores facilitate the transportation of gas molecules following Knudsen diffusion within the mesoporous matrix, 54−56 which is particularly favorable for the guest molecules efficiently interacting with the pore wall to achieve improved catalytic activity. 57,58 In this work, a highly efficient and stable SMC catalyst was constructed by confinement of ultrafine Pt NCs in the mesopores of mesoporous titania (mpTiO 2 ). The resultant Pt-mpTiO 2 catalysts possess well dispersed Pt NCs and a favorable EMSI effect and exhibited an excellent long-term stability in catalyzing the WGS reaction due to the well retained metal/metal oxide interfaces in the mesoporous matrix during reaction. Strikingly, the Pt-mpTiO 2 catalysts exhibited an unexpected increasing activity during repeated catalyzing of the WGS reaction. Such a self-enhancement phenomenon was found to stem from the continuous formation of Ti 3+ -O v -Pt δ+ active sites at the interfaces of the confined Pt NCs and the TiO 2 pore wall. These novel Pt-mpTiO 2 catalysts were demonstrated to be efficient not only for the WGS reaction but also for other catalytic reactions such as hydrogenation and the photocatalytic reaction. These findings provide new insights for development of nanostructured materials with tailored metal−metal oxide interfacial microenvironments and superior activities.

■ RESULTS AND DISCUSSION
It started from the synthesis of mesoporous TiO 2 (mpTiO 2 -PS x , x refers to the repeating unit number of styrene: 120, 173, or 248) supports via the solvent evaporation induced coassembly (EICA) approach (Scheme S1), where lab-made amphiphilic poly(ethyl oxide)-block-polystyrene (PEO-b-PS x ) diblock copolymers and tetrabutyl orthotitanate (TBOT) were used as the template (the porogen) and titania precursor, respectively. During the assembly process, the hydrophobic PS segments can aggregate as a micellar inner core, and the hydrophilic PEO segments interact with hydrolyzed titanium precursor during the solvent evaporation induced coassembly process. In addition, the pore size of mesoporous TiO 2 can be flexibly regulated by changing the molecular weight of PS segments. Thanks to the unique structure-directing effect of the PEO-b-PS x template, the obtained mpTiO 2 has interconnected uniform spherical mesopores (pore size 11.2, 14.9, and 24.6 nm, respectively), a high specific surface area (140 m 2 /g, 113 m 2 /g, and 92.3 m 2 /g, respectively), and a polycrystalline pore wall made of anatase TiO 2 nanocrystals (Figures S1 and S2 and Table S1). For comparison, nonporous anatase TiO 2 (npTiO 2 ) was also synthesized via a similar procedure without adding PEO-b-PS x . The npTiO 2 powder ( Figure S3) has irregular structure with a low specific surface area (2.94 m 2 /g) due to the uncontrolled sintering and aggregation of TiO 2 NPs without the structure-directing effect of the PEO-b-PS x template. 59,60 After loading Pt NCs into the mesopores using chloroplatinic acid hexahydrate as a precursor, novel SMC catalysts can be obtained, and the ordered mesoporous structure is well-retained for the Pt-mpTiO 2 -PS 120 , as confirmed by electron microscopy observation ( Figure 1A−C and Figure S4), N 2 adsorption−desorption isotherms ( Figure S1 and Table S1), X-ray diffraction (XRD) ( Figure S5), and small-angle X-ray scattering (SAXS) measurements ( Figure S1). The thermogravimetric analysis (TGA) result shows that the mpTiO 2 obtained after calcination in air at 450°C for 30 min has no visible weight loss at temperatures above 450°C in the air ( Figure S6), and it confirms that residual carbon has been completely removed after calcination in the air. Notably, this facile in-pore deposition method allows for a convenient synthesis of Pt-mpTiO 2 catalysts with a high loading amount of Pt NCs up to 1.72 wt % (Table S2), and the obtained ultrafine Pt NCs with a size of 1.06 ± 0.06 nm ( Figure 1D) are homogenesouly distributed in the mesopores of the mpTiO 2 , due to the high specific surface area and favorable confinement effect of mpTiO 2 -PS 120 (Figures 1E,F). Aberration-corrected high angle annular dark-field scanning transmission electron microscopy (ac-HAADF-STEM) images ( Figure 1E) reveal that the Pt NCs in Pt-mpTiO 2 -PS 120 consist of many Pt atoms. The formation of such a unique nanocluster structure of Pt is possibly due to the hydrophilic rough pore wall of mpTiO 2 formed by TiO 2 nanocrystals, which helps to spread the Pt precursor solution but prevent Pt species from aggregation during reduction. 61 By contrast, much larger Pt particles of 3.38 ± 0.72 and 3.25 ± 0.73 nm, respectively, were obtained when the as-synthesized npTiO 2 and commercial anatase TiO 2 (comTiO 2 ) powders were used as the supports ( Figures S3  and S7). These results clearly indicate that mpTiO 2 as the supports have unique advantage for preparing SMC catalysts with highly dispersed Pt NCs. Moreover, the electron energy loss spectrum (EELS) of Pt-mpTiO 2 -PS 120 collected at spots 1, 2, 3, 4, and 5 ( Figure 1G−I) shows that signals of the Ti Ledge and O K-edge were observed at spots 2, 3, and 4, similar to those at spot 1, while no Ti L-edge and O K-edge signals were observed at spot 5. This result demonstrates a partial encapsulation of Pt NCs by mpTiO 2 -PS 120 , indicating a favorable medium MSI for Pt-mpTiO 2 -PS 120 .
The WGS reaction was used as a model to investigate the catalytic behavior of these Pt-TiO 2 catalysts (Figure 2 and Figures S8 and S9), which represents an important energy conversion technology in the chemical industry. The Pt-npTiO 2 shows a poor catalytic activity with a CO conversion of 49.5% at 250°C (Figure 2A, b 1 ). In contrast, the Pt-mpTiO 2 -PS 120 exhibits excellent catalytic activity, and the CO conversion reaches 94.8% at 250°C (Figure 2A, a 1 ). Remarkably, different from the continuously decreased activity of the used Pt-npTiO 2 during the cycling test ( Figure 2A, b 1 − b 4 ), the activity of used Pt-mpTiO 2 -PS 120 increases significantly with a CO conversion of 99.1% at 250°C as a contrast to the fresh one, indicating an unusual self-enhancement of catalytic activity ( Figure 2A, a 1 −a 5 ). Such an unexpected behavior is dramatically different from conventional catalysts whose activities usually decrease during the cycling test. Moreover, in comparison with Pt-npTiO 2 , Pt-mpTiO 2 -PS 120 exhibits wellmaintained CO conversion (∼95.0%) at 250°C within eight cycles for about 96 h ( Figure 2B). After the WGS reaction, the mesoporous structure ( Figure 2C and Figures S10 and S11) and crystal phase ( Figure S12) of Pt-mpTiO 2 -PS 120 are well preserved, indicating good stability. The high dispersion state of Pt NCs was well preserved ( Figure 2C,E), and the mean diameter is 1.07 ± 0.10 nm ( Figure 2D), very close to that (∼1.06 nm) of fresh Pt-mpTiO 2 -PS 120 . According to the EELS spectroscopy of the used Pt-mpTiO 2 -PS 120 collected at spots 1, 2, 3, 4, and 5 ( Figure 2F−H), a partially encapsulated structure of Pt NC by mpTiO 2 -PS 120 is also found, indicating a wellretained medium encapsulation state of Pt NCs. All of these results clearly demonstrate an excellent stability of Pt-mpTiO 2 -PS 120 due to the preservation of medium MSI between Pt NCs and mpTiO 2 support.
X-ray photoelectron spectroscopy (XPS) measurements reveal that the used catalyst has a higher metallic Pt (Pt 0 / Pt 0 + Pt 2+ + Pt 4+ ) ratio (0.56) than the fresh one (0.47) due to the reduction of in situ generated H 2 during the WGS reaction ( Figure 3A and Table S3). Moreover, the Pt NCs in the used catalyst after a long-term WGS reaction for about 96 h only slightly increase to 1.20 ± 0.20 nm (Figure S11), indicating a good long-term stability of the Pt-mpTiO 2 -PS 120 catalyst. Such firmly supported Pt NCs are beneficial to maintaining the EMSI between Pt NCs and mpTiO 2 . It is worth noting that the obtained Pt-mpTiO 2 -PS 120 catalyst exhibits outstanding catalytic activity of up to 13.5 times higher than previously reported similar catalysts working at the same temperature (Table S4). Similarly, both Pt-mpTiO 2 -PS 173 and Pt-mpTiO 2 -PS 248 synthesized using PEO-b-PS templates with higher molecular weights have a large pore size of 12.0 and 20.8 nm, respectively, and they also show much better activity compared to Pt-npTiO 2 ( Figure S8). The CO conversion of the WGS reaction over the three mesoporous catalysts at 250°C follows the sequence Pt-mpTiO 2 -PS 173 > Pt-mpTiO 2 -PS 120 > Pt-mpTiO 2 -PS 248 , consistent with the order of preexponential factor (A) obtained from the kinetic studies (Table S5). Notably, the extent of self-enhancing activity decreases slightly with the increasing pore size of the mpTiO 2 supports possibly due to the weakened confinement effect of the pore wall ( Figure S9). Particularly, the activity of Pt-mpTiO 2 -PS 248 with larger Pt NPs (2.01 ± 0.34 nm) decreases slightly during the cycling test because larger Pt particles can lead to a stronger MSI and further encapsulation of Pt NPs during the WGS reaction ( Figure S4).
To gain insight into the unexpected cyclic performance and excellent stability of the Pt-mpTiO 2 catalysts, XPS measurements were performed on the fresh and used Pt-mpTiO 2 -PS 120 catalysts ( Figure 3A and Table S3). The Ti 2p spectra show that the relative content of Ti 3+ (∼458.3 eV) increases dramatically from 0.48 for the fresh Pt-mpTiO 2 -PS 120 to 0.64 for the cyclic used catalysts, indicating the reduction of mpTiO 2 support during the WGS reaction. 36,62 The O 1s spectra show that the concentration of surface adsorbed oxygen species at oxygen vacancy (O v ; ∼529.4 eV) for the used catalysts increases along with the dramatic down-shift of its binding energy, implying an increased concentration of positively charged O v in the used Pt-mpTiO 2 -PS 120 . 36,63 In addition, the electron paramagnetic resonance (EPR) measurements were also conducted to identify the concentration of O v ( Figure S13). In comparison with the fresh Pt-mpTiO 2 -PS 120 , the signal at a g value of 2.003 that is ascribed to the surface Ti 3+ defect and single electron O 2 − radical trapped O v for the used catalyst becomes stronger ( Figure S13a,b), 64 further confirming the obvious increase of O v concentration and agreeing well with the above-mentioned XPS results. In the XPS spectra for Pt 4f, 65,66 the bands assigned to Pt 0 (∼70.7 and 74.1 eV) for the used Pt-mpTiO 2 -PS 120 shift to higher binding energy ( Figure 3A), indicating the formation of positively charged Pt (Pt δ+ , 0 < δ < 2) NCs and generation of numerous Ti 3+ -O v -Pt δ+ sites at the Pt/mpTiO 2 interfaces. The formation of Ti 3+ -O v -Pt δ+ sites is due to the charge transfer from ultrasmall Pt NCs to mpTiO 2 , which is a typical electronic perturbation phenomenon in EMSI. However, the above findings and phenomena were not observed for Pt-npTiO 2 (Figures S13c,d and S14 and Table S3) Considering that O v is usually formed after the removal of lattice oxygen from crystalline metal oxides, the increase of O v concentration of mpTiO 2 in this study is probably due to the reduction of in situ generated H 2 during the WGS reaction. To unravel the reason for the increase of O v concentration in the used Pt-mpTiO 2 -PS 120 , H 2 temperature-programmed desorption (H 2 -TPD) and H 2 temperature-programmed reduction (H 2 -TPR) measurements were carried out on different catalysts. In comparison with the fresh Pt-npTiO 2 , the H 2 -TPD profile ( Figure 3B) of the fresh Pt-mpTiO 2 -PS 120 clearly shows a desorption peak for the spillover hydrogen at 607°C, proving the obvious hydrogen spillover effect, which can greatly contribute to the increase of O v concentration in mpTiO 2 . Moreover, a remarkable reduction peak was observed at 323°C for mpTiO 2 support in close interaction with Pt in the H 2 -TPR profile of Pt-mpTiO 2 -PS 120 ( Figure 3C), which indicates the easy reduction of adjacent Ti 4+ to Ti 3+ by the spillover hydrogen. 68,69 Density functional theory (DFT) calculations ( Figure 3D and Figure S15) further verify the superior transfer ability of dissociated hydrogen from small Pt NCs to TiO 2 , and lattice oxygen in the vicinity of Pt NCs confined in mpTiO 2 can be more easily removed with a lower energy barrier of 0.2 eV. Therefore, it is clear that, during the WGS reaction, the strong reducing ability of spillover hydrogen generated at small Pt NCs is the main reason for the increase of O v concentration in Pt-mpTiO 2 -PS 120 . According to the further comparison of the H 2 -TPD profiles for the used and fresh Pt-mpTiO 2 -PS 120 ( Figure 3B), it can be found that the spillover-hydrogen desorption peak at 607°C for the used Pt-mpTiO 2 -PS 120 decreases remarkably with the increase of the desorption peak for the chemisorbed hydrogen at 200−300°C , indicating that the increase of O v concentration in the used Pt-mpTiO 2 -PS 120 can impede the hydrogen spillover and in turn accelerate the H 2 desorption from the Pt NCs. Such an interesting and unusual tandem process is extremely favorable for an efficient WGS reaction.
Since steam is involved in the WGS reaction, the effect of steam on a Pt-mpTiO 2 -PS 120 catalyst was also studied. The EPR measurement result ( Figure S13a,e) shows that the steam treatment hardly increases the O v concentration in Pt-mpTiO 2 -PS 120 . Interestingly, in the O 1s XPS spectra of the steamtreated Pt-mpTiO 2 -PS 120 , the peak ascribed to lattice oxygen (∼530.1 eV) shifts to high binding energy, implying that the lattice oxygen is positively charged by bonding with a hydrogen radical (H*) derived from H 2 O dissociation on the adjacent O v ( Figure S16). In situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) spectra of CO adsorption at 25°C for the Pt-mpTiO 2 -PS 120 ( Figure S17) show that the Ti−O band (710 cm −1 ) displays a red-shift (696 cm −1 ) and becomes weaker with steam pretreatment, indicating that the Ti−O bond is more unstable after steam pretreatment. These results obviously confirm that the H* can greatly activate the lattice oxygen of mpTiO 2 to generate active oxygen species that can be easily reduced to O v by spillover hydrogen during the WGS reaction, agreeing well with H 2 -TPR, H 2 -TPD, and DFT calculation results. The lower desorption temperature of spillover hydrogen (460°C) for the steam-treated Pt-mpTiO 2 -PS 120 in contrast with that (607°C) for the fresh Pt-mpTiO 2 -PS 120 ( Figure 3B) indicates the timely desorption of H 2 due to the existence of hydroxyl derived from H 2 O dissociation, 70 thus resulting in superior catalytic activity ( Figure S16). According to the results mentioned above, it can be concluded that during the WGS reaction over Pt-mpTiO 2 -PS 120 ( Figure  3E), H* derived from H 2 O dissociation at the O v sites can activate the adjacent lattice oxygen at the Pt/mTiO 2 interfaces and promote the increase of O v concentration by cooperating with the strong reducing ability of spillover hydrogen, and the newly generated O v nearby the Pt/mTiO 2 interfaces is more active. 71 The newly generated O v would successively promote the dissociation of H 2 O molecules and stabilization of Pt NCs, further facilitating the desorption of generated hydrogen and accelerating the WGS reaction process.
To further study the catalytic mechanism of the Pt-mpTiO 2 -PS 120 catalyst and clarify the superior catalytic performance of the used catalyst during the WGS reaction, in situ timeresolved carbon monoxide diffuse reflectance infrared Fourier transform spectroscopy (CO−DRIFTS) measurements were performed. According to in situ time-resolved DRIFTS spectra of CO chemisorption on Pt-mpTiO 2 -PS 120 at 180 and 210°C ( Figure 4A and Figures S18−S24), it can be concluded that the WGS reaction over the Pt-mpTiO 2 -PS 120 catalyst exclusively underwent a redox pathway rather than an associative pathway over the Ti 3+ -O v -Pt active sites. Interestingly, compared to the fresh Pt-mpTiO 2 -PS 120 , the band intensity corresponding to linearly adsorbed CO on Pt δ− (1950 cm −1 ; Figure 4Ba) decreases along with the increase of the linearly adsorbed CO on Pt 0 (2060 cm −1 ) after the CO adsorption on used Pt-mpTiO 2 -PS 120 was flushed with He for 10 min, indicating the change of charge environment around Pt NCs. This result is in good agreement with the appearance of more Pt δ+ species shown in the XPS spectra ( Figure 3A) and indicates the formation of the Ti 3+ -O v -Pt δ+ active sites in the used Pt-mpTiO 2 -PS 120 . The characteristic bands of adsorbed CO disappear more rapidly with the introduction of H 2 O at 210°C (Figure 4Ba), suggesting the timely transfer of activated CO from used Pt-mpTiO 2 -PS 120 due to the relatively weak CO chemisorption on the Pt δ+ NCs caused by the decreased back-donation of Pt d electrons into the 2π* antibonding orbital of CO. 72 Moreover, the characteristic bands for Ti 3+ −OH increase obviously for the used catalyst (Figure 4Bb For comparison, in situ time-resolved DRIFTS spectra of CO chemisorption over Pt-npTiO 2 were collected (Figures S18−S21 and Figures S25−S28), which disclosed the carboxyl associative pathway of the WGS reaction over Pt-npTiO 2 . The weakened bands for adsorbed CO and intermediates after introduction of H 2 O over the used catalyst ( Figures S25−S28) reveal their decreased ability for CO activation and H 2 O dissociation due to the unfavorable overencapsulation of large Pt NPs induced by the SMSI, and this is further confirmed by the EELS analysis, which shows obvious signals of Ti L-edge and O K-edge at spots 2, 3, 4, and 5, similar to those at spot 1 ( Figure S29). Therefore, a decreasing cyclic activity and stability toward the WGS reaction was observed, although there is no significant change in the size of Pt NPs ( Figure S3).
The unique interfacial structure and catalytic behavior of Pt-mpTiO 2 -PS x catalysts encourage us to extend their application in other heterogeneous catalysis reactions. Furfural is an indispensable intermediate for the sustainable preparation of high value-added platform molecules from biomass (cellulose). The effective transformation and removal of its C−O bond affects directly the purity and selectivity of the synthesized monomers, which is a key step for biomass upgrading and utilization, and the selective hydrogenation of furfural is strongly dominated by the distribution of active sites. 73−75 The Pt-mpTiO 2 -PS 120 displays better catalytic ability and stability than the Pt-npTiO 2 catalyst toward the hydrogenation of furfural to 2-methylfuran (2-MF; Figure 5 and Table S6). During the continuous activity evaluation process, stable catalytic activity with high conversion (∼90%) of furfural and high selectivity (∼80%) toward the target product 2-MF were observed for the fresh Pt-mpTiO 2 -PS 120 catalyst. The result is consistent with the findings in the previous report; 76 namely, Pt nanoclusters (NCs) can selectively activate the C− O bond scission, whereas metallic Pt NPs can promote both C−O activation and ring hydrogenation, thus resulting in a lower selectivity to 2-MF. In addition, the activity of used Pt-mpTiO 2 -PS 120 is further improved with the stability main- tained, and the furfural conversion and selectivity toward 2-MF reached as high as ∼100% and ∼90%, respectively, which are superior to most supported metal catalysts reported in the previous work under similar test conditions, even bimetallic catalysts (Table S7). The decreased electron density of these Pt NCs due to the EMSI promotes the furfural adsorption and contributes to the improved activity. Moreover, the used Pt-mpTiO 2 -PS 120 after the WGS reaction also possesses superior catalytic ability in photocatalytic degradation of organic pollutants ( Figure S30) with almost twice the degradation rate compared to the fresh one.

■ CONCLUSIONS
In summary, a highly stable supported metal cluster (SMC) catalyst with rich active Pt/mpTiO 2 interfaces was designed by confinement of Pt NCs in the uniform mesopores of mesoporous titania, and an unusual self-enhancing activity was discovered for this novel SMC catalyst. The self-enhancing activity of the Pt-mpTiO 2 was found to stem from the in situ generated Ti 3+ -O v -Pt δ+ active sites that play important roles in facilitating the dissociation of H 2 O, the transfer of activated CO, and the desorption of generated hydrogen and eventually significantly accelerate the WGS reaction via the redox pathway. It is the collective effects of the strong reducing ability of spillover hydrogen over Pt NCs, the easy activation of lattice oxygen on mpTiO 2 , and the electronic metal−support interaction that allow for the formation of Ti 3+ -O v -Pt δ+ active sites at the metal/metal oxide interface. The confinement effect of mpTiO 2 support contributes to the stable dispersion of ultrafine Pt NCs and prevents their migration and growth, resulting in an outstanding catalytic stability in the WGS reaction with activity of up to 13.5 times higher than reported similar catalysts. The Pt-mpTiO 2 catalysts also show superior performance toward the selective hydrogenation of furfural to 2-methylfuran compared to Pt-npTiO 2 . The discovery of the self-enhancing activity and the findings about the underlying mechanism of mpTiO 2 supported Pt NCs can serve as a useful guideline in exploring a variety of supported metal nanoclusters with improved interfacial activity in different fields, including catalysis, sensing, energy conversion, and storage. ■ EXPERIMENTAL SECTION Chemicals and Materials. All of the chemicals were analytical grade. Monomethyl poly(ethylene oxide) (M w : 5000 g·mol −1 ), chloroplatinic acid hexahydrate (H 2 PtCl 6 ·6H 2 O), furfural, Rhodamine B, and the commerical TiO 2 (anatase phase) were purchased from Aladdin Chemical Reagent Co. Ltd. Tetrabutyl orthotitanate (TBOT), tetrahydrofuran (THF), and ethanol (EtOH) were purchased from Sino-Pharm Chemical Reagent Co. Ltd. Deionized water was used in the whole experimental process.
Synthesis of Mesoporous TiO 2 Support. The amphiphilic poly(ethyl oxide)-block-polystyrene diblock copolymers with different hydrophobic chain length (PEO-b-PS x ) templates prepared by the atom transfer radical polymerization (ATRP) method were used for the synthesis of mesoporous TiO 2 (mpTiO 2 ) via the solvent evaporation induced coassembly (EICA) approach (Supplementary Scheme 1, steps 1−4). In a typical synthesis process, 100 mg of PEO-b-PS was first dissolved in 5 mL of THF to form a homogeneous solution, and 150 μL of concentrated hydrochloric acid (35%− 37% HCl) and 150 μL of concentrated acetic acid were added dropwise into the above solution under magnetic stirring. After stirring for 5 min, 400 μL of TBOT was added dropwise into the resultant solution. After stirring for another 2.0 h, the solution was poured into Petri dishes to evaporate solvent at 25°C for 24 h, followed by sequential heating at 40°C for 24 h to remove the solvent completely and at 100°C for another 24 h to fix the structure. The transparent film was scraped and crushed into faint yellow powders which were first calcined in a tube furnace under a N 2 atmosphere at 350°C for 3 h with a heating rate of 1°C/min and then in the air at 450°C for 30 min with a heating rate of 5°C/min. By using the PEO-b-PS template with different PS lengths (PEO-b-PS 120 with Mn = 17484 g/mol and polydispersity index (PDI) = 1.13, PEO-b-PS 173 with Mn = 23038 g/mol and PDI = 1.11, and PEO-b-PS 248 with Mn = 30797 g/mol and PDI = 1.12), crystalline ordered mesoporous TiO 2 with different pore sizes can be obtained and denoted as mpTiO 2 -PS x , wherein x refers to the polymerization degree of the PS segment. For comparison, nonporous TiO 2 (designed as npTiO 2 ) was synthesized according to the same procedure but without adding the template.
Synthesis of Pt-mpTiO 2 Catalyst. Pt-mpTiO 2 catalysts were prepared by the wet-impregnation method (Supplementary Scheme 1 step 5). Typically, 0.1 g of mpTiO 2 -PS x was dispersed into 5 mL of H 2 O by sonication. Then, 1 mL of H 2 PtCl 6 ·6H 2 O aqueous solution (0.01 mM; corresponding to a 2 wt % loading amount of Pt) was added to the mixture, followed by stirring at 25°C for 12 h. Finally, the mixture was centrifuged, and the product was washed with H 2 O two times followed by EtOH washing two times and vacuum drying at 50°C for 8 h. For comparison, the Pt-npTiO 2 catalyst was also prepared via the above procedure. Subsequently, the obtained samples were treated in a H 2 /Ar atmosphere (1:19, v/v) at 300°C for 2 h with a ramp of 5°C/min, followed by cooling to room temperature under a N 2 atmosphere. The obtained catalysts were labeled as Pt-mpTiO 2 -PS x and Pt-npTiO 2 , respectively.
Characterization and Measurement. The powder X-ray diffraction (XRD) measurements were conducted using a Bruker D4 X-ray diffractometer (Germany) with Ni-filtered Cu Kα radiation (40 kV, 40 mA), and small-angle X-ray scattering (SAXS) patterns were collected using a Nanostar U SAXS system (Bruker, Germany). Thermogravimetric analysis (TGA) was conducted via a TA Instruments SDT Q600 analyzer (America) from 25 to 600°C with a ramp rate of 10°C /min. Field-emission scanning electron microscopy (FESEM) was performed on the Zeiss Ultra 55 FESEM (Germany). Transition electron microscopy (TEM) and high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were carried out on a Tecnai G2 F20 S-Twin microscope (FEI, America). Size statistics of the Pt NCs or Pt NPs in the catalysts were made using ImageJ software across more than 100 points on the HAADF-STEM image, and the mean value and standard deviation were calculated for comparison. Aberration-corrected high angle annular dark-field scanning transmission electron microscopy (ac-HAADF-STEM) and electron energy-loss spectroscopy (EELS) were carried out on a FEI-Titan Cubed Themis G2 300 (The Netherlands). N 2 adsorption−desorption measurements were carried out at 77 K on a Micromeritics Tristar 2420 analyzer. X-ray photoelectron spectroscopy (XPS) measurements were performed on an AXIS Ultra DLD X-ray photoelectron spectrometer with a MONO Al source (Shimadzu Corp). The electron paramagnetic resonance (EPR) spectra were conducted at room temperature on a Bruker EMX-10/12 spectrometer with 9.5 GHz X-band. H 2 temperature-programmed desorption (H 2 -TPD) was conducted on a Micromeritics AutoChem II 2920 with an online mass spectrometer (MS). H 2 temperature-programmed reduction (H 2 -TPR) was conducted on a Micromeritics AutoChem II 2920 equipped with a thermal conductivity detector (TCD). Loading amounts of Pt in the catalysts were detected on a Thermo Scientific iCAP 7400 inductively coupled plasma atomic emission spectrometer (ICP-AES).
Catalytic Tests for Water−Gas Shift (WGS) Reaction. The WGS reaction was performed on a continuous flow fixedbed reactor from 120 to 300°C under atmospheric pressure. During each test, 50 mg of catalyst was placed into the U-type quartz tube with an interior diameter of 6 mm. Water was injected into the heated gas feed line (160°C) using a calibrated syringe pump, and the generated steam was mixed with the CO/N 2 gas stream before entering the reactor. The reactant gas consisted of 2% CO, 8% H 2 O, and 90% N 2 with total flow rate of 50 mL/min, and the weight hourly space velocity (WHSV) was maintained at 60 000 mL g cat −1 h −1 . The outlet gas was analyzed online using a gas chromatograph equipped with a thermal conductivity detector (TCD) and a fire ionization detector (FID) after the condensation of water at the exit of the reactor. During each test, the catalyst was pretreated in a N 2 atmosphere at 300°C for 30 min and then cooled down to room temperature. At each reaction temperature (120, 150, 200, 250, and 300°C), the test was maintained for 30 min. The cyclic catalytic activity evaluations of Pt-mpTiO 2 and Pt-npTiO 2 were conducted continuously five times under the same heating procedures. Catalytic stability evaluation was carried out at 250°C for a total of 96 h with a period of 12 h. During the study, no methane was detected, and the conversion of CO was calculated using the following equation: For the calculation of mass specific activity (k) and metal normalized activity (R), the total CO conversion was kept below 10%. The mass specific activity and metal normalized activity were calculated using the following equations: DRIFTS Experiments. In situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) was conducted using an in situ diffuse-reflectance cell on the Nicolet 6700 (ThermoFisher) equipped with an MCT detector. The detailed process is shown as follows.
In situ time-resolved DRIFTS spectrum analysis was performed to identify the catalytic performance of the interfacial active sites. About 50 mg of catalyst sample was filled into the reactor, pretreated in a He atmosphere at 300°C for 30 min. Then, the sample was slowly cooled to 180 and 210°C , respectively, followed by the chemisorption of CO in a 1% CO/He atmosphere for 10 min and subsequent flushing with He for 10 min. Finally, H 2 O was injected into the reactor, and the time-resolved DRIFTS spectra were recorded using a 64scan quick sweep mode with a resolution of 4 cm −1 .
In situ DRIFTS spectrum analysis was also performed to reveal the influence of H 2 O on promoting the activity of the mesoporous catalyst. The catalyst sample was placed into the reactor, pretreated in a He atmosphere at 300°C for 1 h or in a H 2 O/He atmosphere at 300°C for 30 min, followed by flushing with He for 30 min. After cooling down to 25°C, 1% CO/He was introduced for 10 min. Then, the DRIFTS spectra were collected after flushing with He for 10 min.
DFT Calculations. We have employed first principles 77,78 to perform all spin-polarization density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew−Burke−Ernzerhof (PBE) 79 formulation. We have chosen the projected augmented wave (PAW) potentials 80,81 to describe the ionic cores and take valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 450 eV. Partial occupancies of the Kohn−Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10 −6 eV. A geometry optimization was considered convergent when the energy change was smaller than 0.03 eV Å −1 . The vacuum spacing in a direction perpendicular to the plane of the structure is 15 Å. The Brillouin zone integration is performed using 3 × 3 × 1 Monkhorst−Pack k-point sampling for a structure. The free energy was calculated using the equation where G, E, ZPE, and TS are the free energy, total energy from DFT calculations, zero-point energy, and entropic contributions, respectively. Hydrogenation of Furfural. A tandem reactor coupled with GC-MS (Froniter Rx-3050-Agilent7890B-5977B) was used to evaluate the catalytic performance of nascent and used catalysts for furfural hydrogenation. Twenty-five milligrams of the catalyst was placed in the second reactor tube in a flattemperature regime of 300°C; 10 mL of H 2 + 41.5 mL of He continuously passed through the catalyst layer (10 mm in height and 2 mm in diameter) during the whole test. Subsequently, 5 μL of furfural liquid (99% AR, Aladdin) was injected from the top of first reactor and evaporated at 300°C immediately, and it took about 5 s for the furfural vapor to pass through the catalyst completely. The temperature program of the chromatographic column is as follows: initial hold at 40°C for 5 min, then increase to 250°C with a heating rate of 5°C/ min and hold at 250°C for 10 min. The injection interval was 1 h, and the test times was equal to the total time of hydrogen exposure on the catalyst. The stability of the catalyst in a longterm hydrogen atmosphere was evaluated by the selectivity and activity of this transient probe reaction without consideration of catalyst poisoning and carbon deposition.
Photocatalytic Degradation of Rhodamine B. The photocatalytic performance of the Pt-mpTiO 2 -PS 120 catalyst was tested by the model reaction of photocatalytic degradation of Rhodamine B (RHB). A xenon lamp (300 W) coupled with a 420 nm filter was used as the light source. A total of 12.5 mg of the catalyst was added into 25 mL of RhB solution (10 mg/ L), followed by continuous magnetic stirring to keep a homogeneous system. Before the experiment, the reactor was placed in the dark for 4 h to ensure the adsorption equilibrium of RhB on the catalyst materials. During the experiment, the absorbance of RhB solution at 552 nm was tested using a UV− vis spectrometer every 10 min. The degradation rate calculated by the following equations was used to describe the photocatalytic ability of Pt-mpTiO 2 -PS 120 : where C 0 is the initial concentration of RhB, C is the concentration of RhB at time t, A 0 is the initial absorbance of RhB solution, and A is the absorbance of RhB solution at time t.
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